Margin Calculation

I am trying to calculate the average margin.

I have two columns: margin for the item and units sold.

For example:

If i sold 50 units at 30% margins and 2 units at 50% margin, my average margin would be roughly 31%

I need to do a summation of total units and total margin %

If I do not have the summation my average margins would be incorrect and show 40%.

How do I put that calculation in Beast Mode?

Thanks!